The monitoring and control of the condition of fresh foodstuffs, which have been given enhanced, but not indefinite, shelf life by chilling and refrigerated storage, poses a major problem in our health conscious society. Thus the consumer demands fresh food, conveniently packaged, and with all-the-year round availability in shops and supermarkets, while at the same time insisting that there is no danger of spoilage leading to impaired looks or flavour, or, more importantly, health risk, in the purchased foods.
Oxidation of lipids, nucleotide degradation by endogenous enzymes and bacterial growth processes involving the metabolism of a wide range of food constituents including carbohydrates and amino acids can all contribute to losses in sensory qualities of foods and their ultimate rejection by the consumer. The relative importance of these various spoilage processes may vary from product to product, with conditions of transportation and storage, with intended use etc., but the consequences of bacterial growth are commonly an important contributory factor. This is especially the case for chilled-fresh foods.
Determinations of the extent and/or precise nature of spoilage may be required for routine quality assurance, for regulatory purposes or for establishing the cause for any particular condition and hence appropriate remedial action. Sensory and microbiological criteria are traditionally used for these purposes, but both have their limitations. The subjectivity of sensory data poses many problems in routine use, not least because of the difficulties in assigning meaningful criteria for differentiating "acceptable from non-acceptable" (or "spoiled from non-spoiled") product. While total bacterial numbers often bear some relationship to acceptability, there are still problems in their use. Not all bacteria growing on food necessarily contribute to spoilage, nor is there a consistent relationship between bacterial numbers and spoilage. Thus, high pH meat (dark, firm, dry: DFD), spoils at lower cell densities than normal-pH meat under identical storage conditions whereas, on vacuum-packaged meat, bacterial numbers may reach a maximum and then remain unchanged for significant periods without obvious sensory change.
Measurement of chemical change, whether of microbial or non-microbial origin, offers a possible alternative to microbiological and sensory criteria. For example, an electrochemical method for glucose and enzyme sensors for nucleotide degradation products and for certain amines have already been proposed.
It has been found by Dainty et al., J. Appl. Bact., (1985), 59, 303-309 and J. Appl. Bact., (1989), 66, 281-289 that the volatile compound diacetyl (butane 2,3-dione) is produced during chill storage of red meats with free access to air (through a permeable film overwrap) or in high O.sub.2 /CO.sub.2 gas atmospheres, and that diacetyl has a high potential as an index of freshness/acceptability/spoilage. This compound has been detected consistently during storage, and at relatively early stages when sensory changes were restricted to development of staleness rather than obvious spoilage. Evidence indicating that accumulation may also occur in vacuum packs has also been suggested as a measure of the condition of some citrus fruit juices and the sanitary status of the processes used in their manufacture, and as a means of detecting supplementation of aroma constituents to mask organoleptic defects in butter. Diacetyl has also been employed as an additive in the manufacture of margarine and cheese. Additionally it is produced during yeast fermentation and plays a role in the flavouring of beer. Sensitive, rapid methods for detection and analysis of diacetyl, which can be applied on- or at-line, to monitor food manufacturing processes and quality deterioration during transportation and storage, would therefore be of great value, as such methods would be non-destructive and would minimize the potential hygiene problems associated with the invasive sampling needed for analysis of non-volatile compounds. Such methods have not so far been available.